Process for producing semiconductor device

ABSTRACT

A semiconductor device fabricating method includes forming an amorphous silicon film on a substrate irradiating the amorphous silicon film with laser light to transform at least a part of the amorphous silicon film into a polycrystalline silicon film, and oxidizing the surface of the polycrystalline silicon film in an atmosphere including oxygen, after the irradiation. The laser light is a linear beam having an energy-density gradient of at least 3 (mJ/cm 2 )/μm in a widthwise direction, and the linear beam is generated by transforming pulsed laser light with a wavelength in a range between 350 nm and 800 nm. The oxidation is performed in a saturated water vapor ambient at a pressure of at least 10 atmospheres and at a temperature in a range between 500° C. and 650° C. With this method, a semiconductor device with excellent crystallinity can be easily fabricated.

TECHNICAL FIELD

The present invention relates to a method for fabricating asemiconductor device and, more particularly, to a method for fabricatinga thin film semiconductor.

BACKGROUND ART

A polycrystalline silicon thin film transistor formed on an insulatingsubstrate generally has a MOS-FET (Metal Oxide Semiconductor-FieldEffect Transistor) structure. A common method for fabricating this thinfilm transistor is a method which forms a semiconductor layer ofpolycrystalline silicon on an insulating substrate, then laminates asilicon oxide film as a gate insulating film on the polycrystallinesilicon film by a chemical vapor deposition (CVD) method, and furtherforms a gate electrode thereon. In a polycrystalline thin filmtransistor fabricated according to this method, there may locally existcrystal defects, impurities and the like on the surface of thepolycrystalline silicon which forms the MOS interface. Particularly, ifthere exist crystal defects and the like in the region between a sourceregion and a drain region formed at portions including the surface ofthe polycrystalline silicon film, this induces problem of reduction inan electron mobility or a positive hole mobility in this region or risein a threshold voltage.

Japanese Patent Laying-Open No. 11-67758 discloses a fabricating methodincluding a process for oxidizing a polycrystalline silicon film in anatmosphere mainly consisting of oxygen and providing a low oxidationrate. This fabricating method can cause the surface layer of thepolycrystalline silicon film to be slowly oxidized; thereby decreasingcrystal defects, making the film quality uniform, and alleviatingirregularities on the surface of the polycrystalline silicon film whichis left unoxidized. Also, by combining with an oxidation process in anatmosphere mainly consisting of water vapor and providing a highoxidation rate, the rate of oxidation inside the polycrystalline siliconfilm can be increased and a high-quality semiconductor film includingfew crystal defects can be formed. Further, when the two processes areperformed in an atmosphere under a pressure in a range of 1 to 50atmospheric pressures and at a temperature in a range of 300 to 700° C.,the formation of an oxide film, and the like can be efficiently achievedand damage of the insulating substrate can be prevented. In this priorart, as the method for forming a polycrystalline silicon film, a furnaceannealing method (solid phase growth method), a laser annealing method(melting recrystallization method), or the like is employed.

Japanese Patent Laying-Open No. 9-312403 discloses a fabricating methodwhich retains nickel elements in contact with a certain region of anamorphous silicon film. Heat treatment is applied to the amorphoussilicon film, on which the nickel elements have been placed, to causecrystal growth in a direction parallel to the substrate. Heat treatmentis further applied thereto in an oxidative atmosphere including halogenelements to form a thermal oxide film. Then, a thin film transistor(TFT) is fabricated such that the aforementioned crystal-growthdirection is coincident with the direction connecting the source anddrain regions. This fabricating method enables to provide a thin filmtransistor having an excellent mobility and an S-value as transistorcharacteristics.

-   Patent Document 1: Japanese Patent Laying-Open No. 11-67758 (pages 3    to 6, FIGS. 1 to 4)-   Patent Document 2: Japanese Patent Laying-Open No. 9-312403 (pages 4    to 10, FIGS. 1 to 5)

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

With the fabricating methods disclosed in Japanese Patent Laying-OpenNos. 11-67758 and 9-312403, the MOS interface can be formed inside thepolycrystalline silicon film. This enables to form a MOS interfaceincluding few crystal defects and impurities existing on the surface ofthe polycrystalline silicon, thereby providing a thin film transistorwith excellent transistor characteristics.

However, the fabricating method disclosed in Japanese Patent Laying-OpenNo. 11-67758 employs, in order to form a polycrystalline silicon film, ageneral method using laser annealing for transforming an amorphoussilicon film into a polycrystalline silicon film by irradiating theamorphous silicon film with laser light. With this method, crystalgrowth in the polycrystalline silicon is caused from the lower side ofthe molten silicon film, namely, the insulating substrate-side thereofexisting at the deepest portion and having a lowest temperature. Sincecrystal grows from the lower side towards the upper side of the siliconfilm, namely, towards the surface thereof, the closer to the surface ofthe silicon, the more favorable crystal will be formed. When the surfaceof the polycrystalline silicon film is oxidized in order to form a gateinsulating film and the like, in the polycrystalline silicon film, theportion having excellent crystallinity will become a silicon oxide film.On the other hand, although the MOS interface at which the source regionand the drain region are to be formed is maintained clean, it will beformed at a portion inside the polycrystalline silicon film having poorcrystallinity. Since the semiconductor layer is formed at a portionhaving poor crystallinity in the polycrystalline silicon film, there hasbeen a problem that the performance of the thin film transistor is notsufficiently improved.

The fabricating method in Japanese Patent Laying-Open No. 9-312403causes crystal growth in a direction parallel to the main surface of theinsulating substrate and, then, forms a silicon oxide film by oxidizingthe surface of the polycrystalline silicon film. Therefore, it is deemedthat there will be no significant variation in the crystallinity alongthe thickwise direction of the polycrystalline silicon film. Namely, itis deemed that even when the polycrystalline silicon film is oxidized inorder to form a silicon oxide film, the left silicon film will haveexcellent crystallinity. However, in this fabricating method, in orderto cause crystal growth in a direction parallel to the main surface ofthe substrate, it has been necessary that nickel is contained in thesilicon film or nickel is removed. Thus, there has been a problem ofcomplexity of the processes. Further, there has been a problem ofnecessity for fabricating it in a high-temperature atmosphere at about1000° C.

The present invention was made in order to overcome the aforementionedproblems, and aims to provide a semiconductor device fabricating methodwhich can easily fabricate a semiconductor device with excellentcrystallinity.

Means for Solving the Problems

A semiconductor device fabricating method based on the present inventionincludes an amorphous silicon laminating process for forming anamorphous silicon film on a substrate, an irradiation process forirradiating the amorphous silicon film with laser light to transform atleast a part of the amorphous silicon film into a polycrystallinesilicon film, and an oxidation process for oxidizing the surface of thepolycrystalline silicon film in an atmosphere including oxygen after theirradiation process. Herein, the laser light is a linear beam having anenergy-density gradient of 3 (mJ/cm²)/μm or more in the widthwisedirection, and the linear beam is generated by transforming pulse laserlight with a wavelength in a range between 350 nm or more and 800 nm orless. The oxidation process is performed in an atmosphere of saturatedwater vapor under a pressure of 10 atmospheric pressures or more and ata temperature in a range between 500° C. or more and 650° C. or less.

Effect of the Invention

According to the present invention, a semiconductor device havingexcellent crystallinity can be easily fabricated.

BRIEF DESCRIPTION OF THE DRAWINGS BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view for describing a first step of asemiconductor device fabricating method based on the present invention.

FIG. 2 is a cross-sectional view for describing a second step of thesemiconductor device fabricating method based on the present invention.

FIG. 3 is a cross-sectional view for describing a third step of thesemiconductor device fabricating method based on the present invention.

FIG. 4 is a cross-sectional view for describing a fourth step of thesemiconductor device fabricating method based on the present invention.

FIG. 5 is a cross-sectional view for describing a fifth step of thesemiconductor device fabricating method based on the present invention.

FIG. 6 is a cross-sectional view for describing a sixth step of thesemiconductor device fabricating method based on the present invention.

FIG. 7 is a schematic view of a laser-light irradiation device based onthe present invention.

FIG. 8A is a schematic perspective view of laser-light irradiation inthe semiconductor device fabricating method based on the presentinvention.

FIG. 8B is a perspective view for describing an amorphous silicon filmbeing irradiated with laser light in the semiconductor devicefabricating method based on the present invention.

FIG. 9 is an illustration of laser-light irradiation to an amorphoussilicon film in the semiconductor device fabricating method based on thepresent invention.

FIG. 10A is an illustration of the molten portion being crystallized inthe semiconductor device fabricating method based on the presentinvention.

FIG. 10B is an illustration of the molten portion being crystallized ina semiconductor device fabricating method based on the prior art.

FIG. 11 is a plan view for describing crystal grains in apolycrystalline silicon film formed according to the present invention.

FIG. 12 is an enlarged cross-sectional view of a portion in the vicinityof a MOS interface in a thin film transistor fabricated according to thepresent invention.

FIG. 13 is a graph for describing the mobility of thin film transistorsfabricated according to respective fabricating methods.

FIG. 14 is a graph for describing the threshold voltages of the thinfilm transistors fabricated according to respective fabricating methods.

DESCRIPTION OF SYMBOLS

-   -   1: insulating substrate, 2: amorphous silicon film, 3:        polycrystalline silicon film, 5: silicon oxide film, 6: gate        electrode, 7: protective film, 8: source and drain regions, 10:        Nd:YAG laser second harmonic wave lasing device, 11: variable        attenuator, 12: movable stage, 13: target, 14: linear-beam        shaping optical system, 15: condenser lens, 16: laser light, 20:        molten portion, 21: crystal grains, 22: source and drain        regions, 25, 26: length, 30, 31, 32: laser-light profile, 35:        temperature distribution curve, 40, 41, 42, 43, 44, 45, 46, 50,        51, 52: arrow

BEST MODES FOR CARRYING OUT THE INVENTION First Embodiment

With reference to FIGS. 1 to 14, a semiconductor device fabricatingmethod according to a first embodiment of the present invention will bedescribed.

FIGS. 1 to 6 are cross-sectional views describing an example of asemiconductor device fabricating method based on the present invention.A semiconductor device according to this embodiment is a MOS-FET. FIG. 1is an illustration of an amorphous silicon laminating process. On theupper surface of an insulating substrate 1, an amorphous silicon film 2is formed by a CVD method as shown by an arrow 40. As insulatingsubstrate 1, for example, a glass substrate and the like or a siliconoxide film formed as an underlying film on the upper surface of a glasssubstrate may be employed. In this embodiment, as insulating substrate1, a silicon oxide film with a thickness of 200 nm formed on the uppersurface of a glass substrate by a CVD method is employed. As thematerial of a film on the substrate, amorphous silicon film 2 is formedby an LPCVD (Low Pressure CVD) method to a thickness of 70 nm.

FIG. 2 is an illustration of an irradiation process. Laser light isapplied to the upper surface of amorphous silicon film 2 in thedirection of an arrow 41. The laser light irradiation causes amorphoussilicon film 2 to be heated and molten. When the molten silicon iscooled and solidified, the crystal structure is changed to apolycrystalline structure; thus, a polycrystalline silicon film 3 isformed.

Next, as shown in FIG. 3, polycrystalline silicon film 3 is patternedinto an island shape. FIG. 4 is an illustration of an oxidation process.By placing it in an atmosphere including oxygen, the surface ofpolycrystalline silicon film 3 is oxidized to form a silicon oxide film5 which will form a gate insulating film later. Preferably, theoxidation process is performed in an atmosphere of saturated watervapor, as well as in an oxidative atmosphere.

Next, as shown in FIG. 5, a gate electrode 6 is formed on the uppersurface of silicon oxide film 5. By these processes, the main portion ofthe MOS structure is formed. Then, impurities are implanted into theregions of the surface of polycrystalline silicon film 3 which lie atthe both sides of gate electrode 6 to form a source region and a drainregion. Further, as shown in FIG. 6, a protective film 7 and source anddrain electrodes 8 which form extractor electrodes for the source anddrain regions are formed around gate electrode 6.

FIG. 7 is an illustration of a laser light irradiation device used inthe irradiation process in FIG. 2. As the laser light in thisembodiment, the second harmonic wave of an Nd:YAG laser is utilized. Thelaser light is generated by an Nd:YAG laser second harmonic wave lasingdevice 10. In this embodiment, the laser light has a wavelength of 532nm. As shown by an arrow 42, the generated laser light passes through avariable attenuator 11 and a linear-beam shaping optical system 14 and,then, is directed to a target 13 placed on a movable stage 12. The laserlight is adjusted to a predetermined intensity by variable attenuator 11and converted into a linear beam profile by linear-beam shaping opticalsystem 14. Movable stage 12 is configured such that target 13 can bemoved relative to the laser light. By using this device, laser heattreatment is applied to target 13.

FIGS. 8A and 8B show schematic views for describing the state of theamorphous silicon film being molten when the amorphous silicon film isirradiated with the laser light. The laser light is converted intolinear beam laser light 16 by a condenser lens 15 formed at the outputpart of the linear-beam shaping optical system (see FIG. 7). Linearlaser light 16 is directed to the main surface of amorphous silicon film2. The distribution of the energy density along the width of the laserlight is, for example, the Gaussian distribution. As shown by alaser-light profile 30, the energy density of the condensed laser lightis largest at the central portion in the widthwise direction. The energydensity gradually decreases with increasing distance outward from thecentral portion. As laser light 16 used for the irradiation, laser lightwith an energy-density gradient of at least 3 (mJ/cm²)/m or more in thewidthwise direction is employed. The energy density is constant in thelongitudinal direction of laser light 16. Thus, the laser light forirradiation has a so-called top-flat shape.

When the second harmonic wave of an Nd:YAG laser is directed toamorphous silicon film 2, amorphous silicon film 2 is heatedsubstantially uniformly in the thickwise direction since amorphoussilicon film 2 has a low absorption coefficient with respect to thesecond harmonic wave. As shown by a temperature distribution curve 35,along the widthwise direction of laser light 16, the portion ofamorphous silicon film 2 corresponding to the portion of laser lightprofile 30 having the largest energy density is heated to a highesttemperature, and the temperature gradually decreases along the widthwisedirection of laser light 16. Therefore, as shown in FIG. 8B, amorphoussilicon film 2 formed on the insulating substrate is moltensubstantially uniformly in the depthwise direction; thus, a moltenportion 20 is formed. In the widthwise direction of laser light 16, onlya constant length is molten. In the longitudinal direction of laserlight 16, molten portion 20 is formed along the linear beam. Namely,molten portion 20 is formed along the region which corresponds to theportion of laser-light profile 30 having the highest energy density.

FIG. 9 shows an illustration of irradiation of pulse laser light toamorphous silicon film 2. In the irradiation of the laser light, themovable stage is moved; therefore, amorphous silicon film 2 movestogether with insulating substrate 1 in the direction of an arrow 43. Onthe other hand, the position of the irradiation of laser light is fixed.The irradiation of laser light is performed while the movable stage ismoved in the widthwise direction of the linear beam. For example, inFIG. 8A, the irradiation of laser light is performed while the movablestage is moved in the direction of arrow 43. In FIG. 9, laser-lightprofile 30 represents the energy density during a most recentirradiation of laser light. A laser-light profile 31 and a laser-lightprofile 32 represent energy-density distributions during pastlaser-light irradiation which have been performed in order. In thisirradiation process, the laser light irradiation is performed while themovable stage is shifted by a constant distance at a time in thewidthwise direction of the linear beam. If the distance by which it ismoved at a time is made larger than the width of the linear beam, thelaser light is directed to the same portion only once. On the otherhand, if the distance by which it is moved at a time is made smallerthan the width of the linear beam, the laser light is directed to thesame portion more than once as shown in FIG. 9, thus continuouslytransforming the amorphous silicon film into a polycrystal. Further, bymoving the movable stage during the irradiation of pulse laser light, acertain region of the amorphous silicon film can be entirely transformedinto a polycrystalline silicon film.

FIGS. 10A and 10B show cross-sectional views for describing the behaviorof the molten silicon being cooled and solidified into a polycrystal.FIG. 10A is an illustration in the case of the irradiation of laserlight based on the present invention. As shown in FIG. 8B, amorphoussilicon film 2 on the insulating substrate is molten substantiallyuniformly throughout the thickwise direction. Since there are smalltemperature differences along the depthwise direction of amorphoussilicon film 2 and along the longitudinal direction of the linear beam,crystal grows in the direction of the relative movement of the laserlight, namely, in the lateral direction (one-dimensional growth) asshown by an arrow 45. Therefore, the crystal grains grow such that theirlongitudinal direction is the lateral direction shown by arrow 45,namely, a direction parallel to the main surface of insulating substrate1. Further, the crystal has no dependence on the depth and apolycrystalline silicon film with excellent crystallinity throughout theentire depth can be obtained.

FIG. 10B shows an illustration of crystal growth in the case oflaser-light irradiation based on the prior art. In fabricating methodsaccording to the prior art, laser heat treatment has been performed witha linear beam using an excimer laser (a typical excimer laser is an XeCllaser with a wavelength of 308 nm). In the case of using an excimerlaser, amorphous silicon has an extremely large absorption coefficientwith respect to the laser light, and most of the laser light is absorbedby an amorphous silicon film in the vicinity of the surface of theamorphous silicon film. Therefore, the temperature is higher near thesurface of the amorphous silicon film, while the temperature is lower atlower portions of the amorphous silicon film. Thus, crystal grows in thethickwise direction of the amorphous silicon film. Namely, thelaser-light irradiation based on the prior art causes a temperaturedistribution along the thickwise direction of amorphous silicon film 2;therefore, crystal grows from near the insulating substrate 1 at whichthe temperature is relatively low towards the opposite side, as shown byan arrow 44. Consequently, the closer to the surface of the amorphoussilicon film, the more excellent crystallinity the formed polycrystalwill have. However, the portion which will be a MOS interface laterexists inside amorphous silicon film 2; therefore, portions with poorcrystallinity will form the semiconductor layer. On the contrary, withthe fabricating method based on the present invention, it is possible toform favorable crystal grains with crystallinity which has no dependenceon the depth in the transformed polycrystalline silicon film aspreviously described.

Since the widthwise distribution of the condensed beam is, for example,the Gaussian distribution, the energy-density gradient of the laserlight applied to the amorphous silicon film varies depending on theposition along the width of the laser light as well as on the energy ofthe laser light. Observations of the shapes of crystal grains infabricated polycrystalline silicon films were made, and the results ofthe observations revealed that energy-density gradients of 3 (mJ/cm²)/μmor more cause crystal growth such that the shapes of crystal grains arelargely biased in the lateral direction.

FIG. 11 shows a plan view of crystal grains in the case there has beensignificant lateral growth. The direction shown by an arrow 50 is thelongitudinal direction of the linear beam and the direction shown by anarrow 51 is the widthwise direction of the linear beam. The laser lightis applied to the amorphous silicon film while being moved relativethereto in the direction shown by an arrow 52. Individual crystal grains21 grow in the lateral direction, namely, in the direction of arrow 51.In this embodiment, there were obtained large crystal grains with grainsizes of about a few μm. More specifically, there were obtained crystalrows of polycrystalline silicon in which length 25 in the lateraldirection, which is the growth direction of crystal grains 21, is twiceor more length 26 perpendicular to the growth direction and thelongitudinal direction of crystal grains 21 is parallel to the widthwisedirection of the linear beam (the direction of movement of the movablestage). By forming such a crystal, a semiconductor film having a largeelectron mobility or positive hole mobility can be provided.Particularly, a semiconductor film having a large mobility in thelongitudinal direction of crystal grains 21 can be provided.

The surface of a polycrystalline silicon film formed according to theaforementioned fabricating method was oxidized in an atmosphere ofsaturated water vapor under a pressure of 20 atmospheric pressures(2.026 MPa) and at a temperature of 600° C. to form an gate insulatingfilm; thus, a thin-film transistor was fabricated. In thisspecification, the method for forming a silicon oxide film by oxidizingthe surface of a polycrystalline silicon film in an atmosphere ofsaturated water vapor is referred to as an “HPA method”.

FIG. 12 shows an enlarged cross-sectional view of a portion around theMOS interface in a MOS-FET fabricated according to the semiconductordevice fabricating method based on the present invention. A siliconoxide film 5 is formed under a gate electrode 6 and, further, apolycrystalline silicon film 3 is formed thereunder. In FIG. 12, thereare schematically shown crystal grains 21 which have been grown insidepolycrystalline silicon film 3. On the upper surface of polycrystallinesilicon film 3, source and drain regions 22 are formed at the sides ofthe regions which will be in shade when gate electrode 6 is projectedonto polycrystalline silicon film 3. Source and drain regions 22 areformed at both the right and left sides. There are formed, at the sidesof gate electrode 6, source and drain electrodes 8 for establishingconduction with source and drain regions 22. In the irradiation processfor the thin-film transistor, irradiation is performed while the movablestage is moved such that the direction of movement of the movable stage(the widthwise direction of the linear beam) is parallel to thedirection connecting the source region and the drain region, as shown byan arrow 46. Therefore, the crystal grains have been grown such that thelongitudinal direction of the crystal grains is parallel to thedirection connecting the source region and the drain region shown byarrow 46. Further, there is no irregularity in the crystal grains alongthe thickwise direction of polycrystalline silicon film 3, and thecrystal grains of the polycrystalline silicon film which has been formedhave substantially uniform shapes. When the MOS-FET is being driven,electrons or positive holes move between the source and drain regions 22in the direction shown by arrow 46.

FIG. 13 shows a graph for making comparison between n-channel thin-filmtransistors fabricated according to the fabricating method based on theprior art and n-channel thin-film transistors fabricated according tothe fabricating method based on the present invention, in terms of themobility out of the electric characteristics. The fabricating methodbased on the prior art was a method for forming polycrystalline siliconusing an excimer laser in the irradiation process. The horizontal axisrepresents methods for forming the gate electrode and the thicknessesformed by the respective methods. The vertical axis represents themobilities of thin-film transistors fabricated according to therespective fabricating methods. The rightmost points along thehorizontal axis designate thin-film transistors including a siliconoxide film formed only by the HPA method. It can be seen that thefabricating method according to this embodiment (a method using YAG2ωlaser annealing) provides higher mobilities than those achieved by thefabricating method based on the prior art (a method using an excimerlaser annealing).

FIG. 14 shows a graph for making comparison between thin-filmtransistors fabricated according to the fabricating method based on theprior art and thin-film transistors fabricated according to thefabricating method based on the present invention, in terms of thethreshold voltage out of the electric characteristics. The horizontalaxis represents the thicknesses of the gate insulating films which wereformed therein and the vertical axis represents the threshold voltages.The respective fabricating methods are the same as the fabricatingmethods shown in FIG. 13. The leftmost points along the horizontal axisrepresent the thin-film transistors including a silicon oxide filmformed only by HPA method. It can be seen that the fabricating methodaccording to this embodiment (a method using YAG2ω laser annealing)provides threshold voltages lower than those provided by the fabricatingmethod based on the prior art (a method using an excimer laserannealing).

As described above, the adoption of the semiconductor fabricating methodaccording to this embodiment can provide a thin-film transistor having ahigh mobility. Further, a thin-film transistor having a low thresholdvoltage can be provided. It is supposed that these effects areattributable to the favorable shapes and sizes of the crystal grainscaused by the difference in the crystal-growth direction as previouslydescribed. Particularly, it is considered possible to fabricate ahigh-performance thin-film transistor in which the region sandwichedbetween the source region and the drain region includes less grainboundaries and thus has a high mobility, since the longitudinaldirection of the crystal grains is parallel to the direction from thesource region to the drain region. As described above, the fabricatingmethod based on the present invention can provide a semiconductor devicehaving excellent transistor characteristics.

Second Embodiment

With reference to FIGS. 13 and 14, a semiconductor device fabricatingmethod based on a second embodiment of the present invention will bedescribed.

In this embodiment, similarly to the first embodiment, a polycrystallinesilicon film was formed by using a second harmonic wave lasing device ofan Nd:YAG laser with a wavelength of 532 nm as the laser lasing device.Subsequently, as the oxidation process, at first, the surface of thepolycrystalline silicon film was oxidized in an atmosphere of saturatedwater vapor under a pressure of 20 atmospheric pressures and at atemperature of 500° C. to form a silicon oxide film.

Under this oxidation condition, a prolonged process is required in orderto provide an oxide film with a predetermined thickness. Therefore, as afirst semiconductor device in this embodiment, the surface of thepolycrystalline silicon was oxidized to a thickness of only 11 nm and,then, a silicon oxide film with a thickness of 35 nm was laminatedthereon by an LPCVD method to provide a predetermined thickness. Ann-channel thin-film transistor employing this silicon oxide film as agate insulating film was fabricated. Next, as a second semiconductordevice in this embodiment, the surface of the polycrystalline siliconfilm was oxidized to form an oxide film with a thickness of 33 nm and,then, a silicon oxide film with a thickness of 10 nm was laminatedthereon by an LPCVD method. An n-channel thin-film transistor employingthis silicon oxide film as a gate insulating film was fabricated.Further, a thin-film transistor employing a silicon oxide film formedonly by an LPCVD method was fabricated. Further, for these fabricatingmethods, thin-film transistors were fabricated by using an excimer laserbased on the prior art in the irradiation process for forming apolycrystalline silicon film. For the thin-film transistors fabricatedaccording to the respective fabricating methods, measurements ofelectric characteristics were performed in order to investigate thetransistor characteristics. The results are described in FIGS. 13 and14.

FIG. 13 shows the mobilities of the respective semiconductor devices.The horizontal axis in FIG. 13 represents the configurations of the gateinsulating films, and represents, in order from the left, the thin-filmtransistors including the silicon oxide film with a thickness of 58 nmformed by the LPCVD method, the thin-film transistors including thesilicon oxide film which was formed to a thickness of 11 nm byperforming the HPA method for 25 minutes and then laminated by athickness of 35 nm thereon by the LPCVD method, the thin-filmtransistors including the silicon oxide film which was formed to athickness of 33 nm by performing HPA method for 75 minutes and thenlaminated by a thickness of 10 nm thereon by the LPCVD method, and thethin-film transistors including the silicon oxide film with a thicknessof 33 nm which was laminated by performing only HPA method for 75minutes.

As a result, when the laser annealing based on the present invention wasperformed, the thin-film transistors including the silicon oxide filmformed by combining the HPA method and the LPCVD method could haveperformance similar to that of the thin-film transistor including thesilicon oxide film formed only by the HPA method. Further, with thefabricating method based on the prior art (fabricating method using anexcimer laser annealing), the mobility tended to decrease withincreasing use of the HPA method, namely, with increasing use of themethod for oxidizing the polycrystalline silicon surface. On the otherhand, with the fabricating method based on the present invention (themethod using YAG2ω laser annealing), no reduction in the mobility wasobserved even when the HPA method was largely used, so that a thin filmtransistor with large mobility can be provided.

FIG. 14 is a graph showing the threshold voltages of the respectivesemiconductor devices. The horizontal axis represents the thicknesses ofthe gate insulating films, and the vertical axis represents thethreshold voltages. It can be seen that the fabricating method based onthe present invention provided lower threshold voltages as compared withthe fabricating method based on the prior art, regardless of thefabricating method using only the HPA method, the LPCVD method and theHPA method, or only the LPCVD method. In focusing attention on thefabricating methods including the HPA method and even in considerationof the thickness dependence, it can be seen that the fabricating methodbased on the present invention provided lower threshold voltages, whichdeviate from the line indicating the dependence on the thickness of thegate insulating film in the semiconductor devices fabricated using theprior-art excimer laser, as shown by a dot line

Further, measurements of the breakdown strengths, namely, the withstandvoltages were performed by maintaining the source region and the drainregion at the same potential and applying a voltage between theseregions and the gate electrode. As a result, it was revealed that thethin-film transistors fabricated by combining the HPA method and theLPCVD method had higher withstand voltages than those of the thin-filmtransistors fabricated by oxidizing the surface of the polycrystallinesilicon film to a thickness of 33 nm. As described above, by forming asilicon oxide film as a gate insulating film using the HPA method and anLPCVD method, a thin-film transistor with a high withstand voltage canbe provided.

As described in this embodiment, by laminating silicon oxide by achemical vapor deposition method after the oxidation process foroxidizing the surface of the polycrystalline silicon film in anatmosphere including water vapor, the MOS interface can be maintainedclean and a silicon oxide film with a required thickness can be formedin a short time period.

Third Embodiment

In a third embodiment based on the present invention, laser light isapplied to an amorphous silicon film using a third harmonic wave lasingdevice of an Nd:YAG laser, instead of a second harmonic wave lasingdevice of an Nd:YAG laser in the irradiation process in the firstembodiment. The third harmonic wave lasing device generates laser lightwith a wavelength of 355 nm. The configurations other than the laserlight for irradiation, such as the optical system for the laser lightand the movement of the insulating substrate and the amorphous siliconfilm during the irradiation of laser light, are the same as those in thefirst embodiment.

As a result of applying laser annealing to the amorphous silicon film,lateral growth in the widthwise direction of the linear beam wasobserved similarly to in the first embodiment in which laser light witha wavelength of 532 nm was applied. Further, there were formed crystalgrains having large grain sizes of about a few μm.

The surface of the polycrystalline silicon film was oxidized in anatmosphere of saturated water vapor under a pressure of 20 atmosphericpressures and at a temperature of 600° C. to form a gate insulatingfilm. A thin-film transistor including the gate insulating film wasfabricated and subjected to performance tests. This thin-film transistorcould provide excellent performance, similarly to the thin-filmtransistor according to the first embodiment which was fabricated byapplying laser light with a wavelength of 532 nm.

From this embodiment and the first embodiment, it can be said that whenthe generated laser light has a wavelength in a range between 355 nm ormore and 532 nm or less, laterally-grown crystal grains can be obtainedand further a thin-film transistor with excellent performance can beprovided.

Fourth Embodiment

In a fourth embodiment based on the present invention, a Ti:Sapphirelaser lasing device was employed, instead of a second harmonic wavelasing device of an Nd:YAG laser in the irradiation process of the firstembodiment. This laser lasing device is a wavelength-variable lasingdevice and can generate laser light with a wavelength in a range of 700nm to 800 nm. The configurations other than the laser light forirradiation, such as the optical system for the laser light and themovement of the insulating substrate and the amorphous silicon filmduring the irradiation of laser light, are the same as those in thefirst embodiment.

As a result of applying laser annealing to the amorphous silicon film,lateral growth in the widthwise direction of the linear beam wasobserved for any wavelength. Further, there were formed crystal grainshaving large grain sizes of about a few μm.

The surface of the polycrystalline silicon film was oxidized in anatmosphere of saturated water vapor under a pressure of 20 atmosphericpressures and at a temperature of 600° C. to form a gate insulatingfilm. Then, a thin-film transistor including the gate insulating filmwas fabricated and subjected to performance tests. This thin-filmtransistor could provide excellent performance, similarly to thethin-film transistor according to the first embodiment which wasfabricated by applying laser light with a wavelength of 532 nm.

From this embodiment and the first embodiment, it can be said that whenthe generated laser light has a wavelength in at least a range between532 nm or more and 800 nm or less, laterally-grown crystal grains can beobtained and, further, a thin-film transistor with excellent performancecan be provided. Further, from the fact that there were not observedlateral growth in the case of using an excimer laser (for example, anXeCl laser with a wavelength of 308 nm) in the prior art and from theresults of the third embodiment and this embodiment, it can be said thatwhen the generated laser light has a wavelength in a range between 350nm or more and 800 nm or less, laterally-grown crystal grains can beobtained and, further, a thin-film transistor with excellent performancecan be provided.

Fifth Embodiment

In a fifth embodiment of the present invention, similarly to the firstembodiment, a second harmonic wave lasing device of an Nd:YAG laser witha wavelength of 532 nm was employed to form a polycrystalline siliconfilm and, subsequently, in the oxidation process, a gate insulating filmwas formed by oxidizing the surface of the polycrystalline silicon filmin an atmosphere of saturated water vapor under a pressure of 20atmospheric pressures and at a temperature of 500° C.

As a result, the growth rate of the oxide film was significantly reducedas compared with the case of the oxidation condition of the firstembodiment in which the temperature is 600° C. and the pressure is 20atmospheric pressures, thereby increasing the processing time requiredfor providing a gate insulating film with a predetermined thickness.Therefore, it is more preferable that the oxidation process foroxidizing the surface of the polycrystalline silicon film in anatmosphere including water vapor is performed at a temperature of 600°C. or more. On the other hand, a thin-film transistor having performancesimilar to that of the thin-film transistor in the first embodimentcould be provided.

From this embodiment, it can be said that a thin-film transistor withexcellent performance can be provided by setting the temperature in theoxidation process to a temperature in a range between 500° C. or moreand 600° C. or less.

Sixth Embodiment

In a sixth embodiment of the present invention, similarly to the firstembodiment, a second harmonic wave lasing device of an Nd:YAG laser witha wavelength of 532 nm was employed to form a polycrystalline siliconfilm and, subsequently, in the oxidation process, a gate insulating filmwas formed by oxidizing the surface of the polycrystalline silicon filmin an atmosphere of saturated water vapor under a pressure of 10atmospheric pressures (1.013 MPa) and at a temperature of 650° C.

A thin-film transistor fabricated according to this method hadperformance similar to that of the thin-film transistor fabricated underthe laser-annealing condition of the first embodiment in which thetemperature was 600° C. and the pressure was 20 atmospheric pressures.On the other hand, when the temperature was set to above 650° C., thethermal contraction in the insulating substrate was increased, therebycausing defects in patterning during the thin-film transistorfabrication processes. Therefore, it was difficult to fabricate a propertransistor. From the results of the fifth embodiment and thisembodiment, it is preferable that the temperature in the oxidationprocess is in a range between 500° C. or more and 650° C. or less.

From the results of the first embodiment and third to sixth embodiments,it is preferable that the oxidation process for oxidizing the surface ofthe polycrystalline silicon film in an atmosphere including water vaporis performed in an atmosphere of saturated water vapor under a pressureof 10 atmospheric pressures or more and at a temperature in a rangebetween 500° C. or more and 650° C. or less. By oxidizing thepolycrystalline silicon film under this condition, a silicon oxide filmwith a predetermined thickness can be formed in a short time period anda dense silicon oxide film can be formed. Further, thin-film transistorshaving low threshold voltages can be fabricated with high productivity.

The aforementioned embodiments which have been described areillustrative and not limitative in all points. The scope of the presentinvention is defined by claims and not by the above description, andincludes all meanings equivalent to the claims and variations within theclaims.

INDUSTRIAL APPLICABILITY

The present invention can be applied to methods for fabricatingsemiconductor devices. Particularly, the present invention can beadvantageously applied to methods for fabricating thin-filmsemiconductors.

1. A semiconductor device fabricating method comprising: an amorphous silicon laminating process for forming an amorphous silicon film on a substrate; an irradiation process for irradiating said amorphous silicon film with laser light to transform at least a part of said amorphous silicon film into a polycrystalline silicon film; and an oxidation process for oxidizing the surface of said polycrystalline silicon film in an atmosphere including oxygen, after said irradiation process, wherein said laser light is a linear beam having an energy-density gradient of 3 (mJ/cm²)/μm or more in the widthwise direction, and said linear beam is generated by transforming pulse laser light with a wavelength in a range between 350 nm or more and 800 nm or less, and said oxidation process is performed in an atmosphere of saturated water vapor under a pressure of 10 atmospheric pressures or more and at a temperature in a range between 500° C. or more and 650° C. or less.
 2. The semiconductor device fabricating method according to claim 1, comprising a process for further laminating silicon oxide, by a chemical vapor deposition method, on the upper surface of said polycrystalline silicon film which has been oxidized in said oxidation process.
 3. The semiconductor device fabricating method according to claim 1, wherein, in said irradiation process, said amorphous silicon film is irradiated with said laser light such that said widthwise direction is parallel to the direction connecting a source region and a drain region in a thin film transistor to be fabricated.
 4. The semiconductor device fabricating method according to claim 2, wherein, in said irradiation process, said amorphous silicon film is irradiated with said laser light such that said widthwise direction is parallel to the direction connecting a source region and a drain region in a thin film transistor to be fabricated.
 5. A method of fabricating a semiconductor device, the method comprising: forming an amorphous silicon film on a substrate; irradiating said amorphous silicon film with laser light to transform at least a part of said amorphous silicon film into a polycrystalline silicon film; and oxidizing said polycrystalline silicon film in an ambient including oxygen, after the irradiation, wherein the laser light is a linear beam having an energy-density gradient of at least 3 (mJ/cm²)/μm in a widthwise direction, and including generating the linear beam by transforming pulsed laser light having a wavelength in a range between 350 nm and 800 nm, and the oxidizing is performed in a saturated water vapor ambient at a pressure of at least 10 atmospheres and at a temperature in a range between 500° C. and 650° C.
 6. The method according to claim 5, comprising depositing a film of silicon oxide, by chemical vapor deposition, on said polycrystalline silicon film after the oxidizing.
 7. The method according to claim 5, including irradiating said amorphous silicon film with the laser light so that the widthwise direction is parallel to a direction connecting a source region and a drain region in a thin film transistor to be fabricated in the polycrystalline silicon film.
 8. The method according to claim 6, including irradiating said amorphous silicon film with the laser light so that the widthwise direction is parallel to a direction connecting a source region and a drain region in a thin film transistor to be fabricated in the polycrystalline silicon film. 